The invention is directed to a tunable add/drop-drop&continue module and to a tunable drop&continue module.
Existing and future optical transmission systems or, networks, particularly transmission systems or, respectively, networks working according to the WDM principle (wavelength division multiplexing), are usually redundantly designed in order to assure an optimally disruption-free optical signal transmission, i.e., for example, the one and the corresponding, redundant connection within an optical transmission system or network are set up—for dependability reasons—via different, mainly spatially separately conducted optical fibers.
Further ring structures are often provided for the realization of the optical communication network. At transitions between different rings or, respectively, ring transmission networks, “add/drop” or, respectively, “drop&continue” functions are provided with the assistance of which the optical signal to be transmitted is split and is forwarded both in the original ring as well as in the new ring or, respectively, even further signals exhibiting a different optical wavelength or, respectively, frequency are added. Wavelength demultiplexers, optical switches and wavelength multiplexers can be utilized for the purely optical realization of such drop&continue functions.
For realizing add/drop functions, modules are known that are composed of two circulatory with intervening, tunable filters, for example fiber Bragg gratings. With respect thereto, see the product note “Drop-Filter mit Zirkulator und durchstimmbaren Faser-Bragg-Gitter” of Laser2000 GmbH (http://www.laser2000.de). These modules are suitable for the realization of drop&continue functions. However, it is conceivable to supplement such a module with optical splitters and optical switches in order to realize the drop&continue functionality.
U.S. Pat. No. 5,748,349 discloses a “Grating-based optical Add/Drop Multiplexer for WDM optical communication systems” wherein add/drop functions are realized with the assistance of tunable fiber Bragg gratings arranged following an optical circulator. For this purpose, the resonant frequency of the fiber Bragg grating exhibiting a low reflection attenuation at the resonant frequency is tuned to the frequency of the optical signal to be delivered and, as a result thereof, the optical signal supplied into the tuned fiber Bragg grating is nearly completely reflected and back-scattered to the preceding optical circulator. With the assistance of the circulator, the reflected optical signal is conducted to the optical “drop” fiber a further optical splitter and switches are also to be provided here for the realization of a drop&continue function, these causing an additional attenuation of the optical signal to be transmitted.
Figure a shows an “add/drop-drop&continue module” realized according to the Prior Art. It is composed of an optical splitter SP that divides an optical signal OS into two sub-signals DK, CK of approximately equal strength, of a first and of a second optical circulator Z11, Z12, of a tunable optical filter BSF, for example a fiber Bragg grating, and of an optical switch SW. For realizing the drop&continue function, the one signal part DK is conducted over the first and second optical circulator Z11, Z12 with an intervening, tunable optical filter BSF, and the other signal part CK is forwarded via the optical switch SW (illustrated switch position).
Given an add/drop function, the one signal part DK is likewise branched off. Simultaneously, a new optical signal AK with the same wavelength can be inserted via the second optical circulator Z12. As a result of employing the optical splitter SP, the module fundamentally comprises an additional attenuation of at least 3 dB. Corresponding to the plurality of add/drop functions or drop&continue functions, the described add/drop or, respectively, drop&continue module is multiply connected in series, as a result of which the attenuation is additionally considerably increased.